Aeroplane construction

The body, or fuselage as it is generally described, constitutes the nucleus of the completed machine, and at the same time offers the most interesting examples of constructional detail. It may be as well to point out that the term “fuselage” is ordinarily applicable to a body of a machine of the tractor type; the short body of the average “pusher” or propeller aeroplane is termed the “nacelle.”

The material chiefly used in the construction of this component is wood, and there are but very few instances where metal is used.

Fuselage Types.

The different types or methods of construction may be classified in the following order:—

1. Box-girder of four longerons or rails, with cross-struts and wire bracing (Fig. 65).

2. Tail portion of longerons, struts and wiring; in the front portion the wire bracing is dispensed with, being replaced by diagonal wood bracing, to which is screwed either three-ply or sheet aluminium alloy (Fig. 66).

3. In this case wire bracing is entirely dispensed with, the four, and occasionally six, longitudinals being connected together by cross struts or formers cut to the required shape, the whole body being covered with three-ply.

4. Laminated or monocoque type, formed by layers of wood and fabric, crossed alternately and glued together.

Box-Girder Type.

Dealing with each type in greater detail, and in order of classification, the details and methods of manufacture of type 1 may be considered. The longerons are usually of ash or hickory, although latterly silver spruce has come into use for this purpose, this being due to the desire to reduce weight to the absolute minimum.

In the opinion of the writer, a spruce longeron should be of larger section than one of hard wood, for one or two reasons. Spruce is a soft wood, and the outside fibres are far more apt to get damaged by a fitting which has been bolted home with too much pressure, also the corners may get rubbed or knocked off, which all means a reduction in strength. The use of a spruce longeron precludes any sharp bends in the contour of the fuselage, as this wood does not lend itself to bending, although it may be sprung to an easy curve. By disposing joints in the longerons, it is possible to arrange the lengths so that the bend is contained in one portion. This portion can then be of laminated construction, i.e. it can be built up of a number of layers glued together, and clamped to a block of the required shape until the glue has set. In some cases the longerons from the engine mounting to the rear cockpit, where additional strength is necessary, are of ash, while aft of that, to the stern post, spruce is the material.

It is usual, in this country at least, to spindle the rails to one of the sections illustrated by Fig. 67, this spindling or channelling running through from nose to stern post, or the front portion, extending as far as the rear cockpit, is left solid, the tail part only being spindled. This channelling is always stopped at the intersection of the cross-struts with the rails, to provide the abutment for the struts, and the extra material to compensate for any holes necessary for the attachment of the fitting. In the shaping of the rails longitudinally, two methods are available: they may be tapered or gradually diminished from the front to the stern post, or the overall section may be parallel to a point somewhere in the neighbourhood of the pilot’s seat, and from that point diminished to the stern post. The first method is obsolete, as all the fittings vary in size, which makes for undue complication as well as increasing the number of jigs and dies necessary to produce the stampings. The second method only partly obviates this, and the only system which permits of the same size fitting being used right through is that in which the rails are of the same overall section throughout, but this is very rarely used.

Another arrangement consists of keeping the rail of equal thickness for approximately 10 ft. from the engine bearers, and then diminishing in a series of steps to the stern post. By this method only three or four sizes of fittings are necessary. Some fittings are not affected by the taper of the rails, and are made the same size throughout, but in nearly every case the attachment to the rails is accomplished either by bolts or screws. The piercing of the longeron, particularly when this is of spruce, is hardly commendable practice, and certainly in view of the many forms of clip fittings in use appears to be unnecessary. A point which apparently escapes the notice of some designers, is the necessity of some allowance being made for unfair stresses induced by landing shocks and rough handling. There is a tendency to make the tail portion separate from the front, the joint occurring just aft of the rear cockpit, so that in the event of damage due to strains transmitted by the tail skid, this portion can be detached and a new portion substituted, which seems infinitely better than dismantling the whole machine and returning the whole body to the works or depôt. In the design of the body under consideration due regard should be given to the necessity of occasional replacement of a damaged rail. Some fittings afford the utmost facility for this, while others render this procedure a lengthy and difficult operation.

Jointing of Longerons.

A popular method of jointing longerons consists usually of a plain butt joint, clipped with some form of steel tube socket, or by fish-plates flanged to clip the edges of the longerons and bolted through. A spliced joint is sometimes used when timber is not procurable in any great length, this consisting of an ordinary splice from 12 to 18 ins. long, glued and riveted, and afterwards, when the joint is thoroughly set, bound with tape soaked in glue and subsequently doped and varnished. As this is a somewhat lengthy operation the socket method predominates. In modern aeroplanes the size of a longeron rarely exceeds 1½ ins. square, and it will therefore be realized that this construction is all that is possible, as, owing to the slightness of material, no advantage would accrue from the employment of a joint of the halved or scarfed variety.

Diagonal Wood Bracing.

A great deal of the foregoing applies to the second type, so far as the longerons and tail portion are concerned. The diagonal wood bracing is usually of spruce, and is, of course, heavier than wiring. The aluminium or duralumin sheeting has latterly given place to three-ply for the outside covering, which may be ascribed to the saving in weight effected by its use, as a square foot of 20 B.W.G. aluminium, which is the general thickness for this purpose, weighs 8 ozs., while a square foot of 3/32 in. birch three-ply weighs approximately 5 ozs. This gives a saving of 3 ozs. for every square foot of surface covered, and moreover three-ply, properly glued and screwed or copper, nailed to the framework, constitutes by far the better stiffening medium. The disadvantages of this method of construction are: (1) the difficulty of re-truing the front portion should distortion occur; (2) erection is somewhat involved; and (3) it is heavier than the first type, although it affords a more solid mounting for the engine, with a consequent reduction of vibration.

Three-ply Fuselage.

The third system is typical of the method adopted for the series of German Albatross machines. There are few, if any, examples of its use in this country, although prior to the war a few constructors favoured its use, and one successful monoplane of note was so built. The writer is acquainted with one pioneer designer who very strongly believes in this form of construction, and certain later developments in the use of three-ply confirm this view. The advantages of this form of construction are: (1) quickness of production; (2) great strength in a vertical and horizontal direction; (3) the result of the longeron being shot through would not endanger the structure to the same extent as with a wire-braced system. Against this must be balanced the fact that: (1) it entails a considerable increase in weight; (2) is weak under a torsional strain, such as that produced by the combined actions of elevator and rudder; and (3) cannot be trued up in the event of distortion. Examples of this system in pre-war machines are afforded by the Martinsyde and Blackburn monoplanes, although the framework in both cases was so formed as to constitute a lattice girder. The tail portion of the Martinsyde was lightened by cutting away diamond-shaped pieces from each bay.

The formers of the Albatross are extremely simple. In the fore part they are cut from three-ply, while at the rear they are just simple frames composed of laths, reinforced where the longerons occur by three-ply stiffeners. There are six longerons, the two middle ones being fixed slightly more than halfway up each side, which are really longitudinal stringers to prevent the three-ply buckling between the points of attachment.

The Monocoque Type.

The monocoque system originated in France, several constructors having produced machines incorporating this feature. The most successful machine produced on these lines was the Deperdussin, and many will recall the excellent streamline form of the machine exhibited at the 1913 Aero Show. These bodies were built over formers of various sections, which were removed when the glue joining the different layers had set. The resultant shell, which was about four millimetres thick, was then covered with fabric and varnished. Several factors militate against its extensive adoption as a method. It is rather costly, and does not seem to be suited to rapid production. In addition, the attachment of such members as the chassis, wings, and interplane struts, is more complicated. It should be noted, however, that various modern machines are similarly built. The Borel firm produced a machine with monocoque body, this being composed of three-ply covering on ribs running diagonally the length of the body, and although this is not such a lengthy operation as the Dep. system, it has not survived, unless one considers flying-boat construction as its modern version. A slight variation of the monocoque system is used for the bodies of some modern aeroplanes. The framework consists of very small stringers arranged at various points on light formers cut to the fuselage section. To this structure is applied two thicknesses of three-ply in the form of strips about 3¼ ins. wide, each thickness being disposed diagonally in opposite directions, as shown by Fig. 68. This is covered with fabric, the total thickness being no more than 1½ mm., and as this is made up of six layers of wood and one of fabric, the fineness of the ply-wood will be realized.

It should be noted that the ply-wood strips do not completely encircle the formers, but are jointed at the top and bottom, a light longeron being arranged at these points.

A detail which would appear to be of great utility at the present time is the arrangement wherein the nose of the body containing the engine and accessories is a separate unit, and in the event of engine breakdown can be detached and another substituted.

Fuselage Contours.

In the design of the contour of the fuselage the type of the motor used is the determining influence. With the vertical “in line” engine, it is possible to design a slim narrow body, while a rotary or radial engine necessitates an increase in width, which also means increased air resistance. With the Vee type engine, the popular practice is to allow the tops of the cylinders to project through the cowling, which permits of a narrower body than if the width of the body equalled the overall width of the engine. Where a rotary engine is employed and the mounting is of the overhung type, the width of the fuselage may be reduced by allowing the engine to project over the sides, and the cowling carried on an arrangement of formers and stringers, which gradually merges into the main structure, as in Fig. 69. It is apparent that the line of the body and that of the fairing should converge as gradually as possible, as, should this be at all abrupt, there is a distinct possibility that the air flow will take the course indicated in Fig. 70, resulting in a dead air region and inefficiency.

It may be taken generally that the wider the body the greater the weight, for the struts have not only to be made longer but also of greater overall section. The practice in this country is to keep the longerons parallel to the centre line on plan, as far as the rear cockpit, tapering from that point to the stern post in a straight or slightly curved line, as Fig. 71.

This simplifies the fittings, the sockets for the centre plane struts are in line, and the different lengths of fuselage struts necessary reduced to a minimum.

The plan outline of several German machines is shown diagrammatically by Fig. 72. It will be seen that from the nose the body gradually widens out until the maximum width, generally in the vicinity of the front seat, is reached, from where it tapers to the tail. This shape appears to satisfy aero-dynamic requirements more closely than either of the foregoing examples; but in practice the difference is not appreciable, and in any case the reduction of head resistance does not compensate for the additional work.

In side elevation the general practice, with exceptions, is to arrange the top longerons parallel to the line of thrust, i.e. the axis of the motor, as in Fig. 73. This simplifies erection and affords a convenient datum line for truing up.

On the German Rumpler and early Albatross biplanes, the upper longerons are curved, as in Fig. 74, but in the most recent versions of the Albatross they are level with the line of thrust. Fig. 75 illustrates an arrangement where the top rails, from a point some distance along, slope down to the nose. By this method the body weight is kept as low as possible and the engine and accessories rendered more accessible. Although it is usual to terminate the body in a vertical knife-edge, formed generally by the rudder post, another arrangement, typical of the Morane monoplane, finishes in a horizontal edge. The German Fokker, obviously inspired by the French Morane, and the Albatross DI, are similarly terminated. This system of tapering to a horizontal knife-edge is not considered the best arrangement from a strength point of view, the flat angle of the bracing wires permitting a certain amount of movement, eventually resulting in slackness and loss of alignment.